1. Field of the Invention
The invention relates to a method for producing a wiring substrate. More particularly, the invention relates to a method for producing a wiring substrate provided with connecting bumps and wiring patterns by the use of a base made of a metal.
2. Description of the Related Art
As a method of producing a wiring substrate, there is a method in which bumps to be subsequently connected to electrodes of a semiconductor element and a wiring pattern connected to the bumps are formed on one side of a base made of a metal, typically copper, and the base is then dissolved for removal, to thereby produce a wiring substrate, as described in JP 2000-323613 A and JP 2002-83893 A.
For the formation of the wiring pattern on a base, a common method for producing a wiring substrate can be applied. For example, a method for forming a wiring layer having a given pattern by forming an insulating layer on a base, forming via holes in the insulating layer, subsequently forming a plating seed layer on the surface of the insulating layer and on the interior faces of the via holes, carrying out electroplating using the base as an power supply layer for the plating to thereby forming a conductor layer on the surface of the insulating layer and on the interior faces of the via holes, and etching the conductor layer, may be used.
As a method for forming bumps to be connected to electrodes of a semiconductor element by plating during the manufacture of a wiring substrate using a base, there is a method for forming protruding solder bumps by concavely etching a surface of a base at the locations corresponding to bumps, carrying out electroplating of solder using the base as an power supply layer for the plating to thereby fill the concavities formed in the base with the solder, and then dissolving the base for removal.
When the solder bumps are formed in the substrate at the locations corresponding to electrodes of a semiconductor element, for the manufacture of a wiring substrate using the base, high precision bump formation is needed because of the small size of the electrode and the small distance between adjacent electrodes, and the bump to be joined to the electrode has a small contact area with the substrate because of its very small size, which may lead to a problem of joining the substrate with the solder bump.
In the manufacture of a wiring substrate using a base, as described above, heat treatment is carried out in order to, for example, heat and cure an insulating layer during the formation of the insulating layer and a wiring pattern on the base. In the case where a wiring substrate provided with solder bumps is formed, there is a problem that the surface of the solder bump is discolored during the heat treatment. It is believed that this is because copper (Cu) used for the base and tin (Sn) in the solder mutually diffuse, to thereby form a compound phase at the interface between the solder and the base. The discolored solder bump surface impairs the appearance of the bump, and also causes problems of, for example, reduced reliability of the electrical connection between the solder bumps and electrodes.